Cold storage of pegylated platelets at about or below 0° C.

ABSTRACT

A method for storing and using platelets and an associated platelet structure. At least one modified platelet is formed. Each modified platelet includes a platelet and at least one polymerated chemical. Each polymerated chemical includes a polymer covalently bonded directly to the platelet or includes the polymer and a linker molecule such that the linker molecule is covalently bonded to the platelet and the polymer is covalently attached to the linker molecule. The polymer of each polymerated chemical of each modified platelet is polyethylene glycol (PEG) or a PEG derivative. Forming each modified platelet does not include modifying the platelet membrane of each platelet with a glycan-modifying agent. The at least one modified platelet is stored in a temperature range below 0° C. After being stored, the at least one modified platelet may be introduced into a subject to treat a condition related to a reduced platelet function.

CROSS-REFERENCE TO RELATED APPLICATION

This is a Continuation-In-Part of Ser. No. 11/673,287 filed Feb. 9, 2007.

FIELD OF THE INVENTION

The present invention relates to method for storing and using platelets and an associated platelet structure. The present invention also relates to the use of the stored platelets for treating subjects.

BACKGROUND OF THE INVENTION

Transfusion of platelets (a commonly transfused cellular component of blood) is a cornerstone of modern medical care for a number of acute and chronic conditions characterized by either excessive bleeding or insufficiency of endogenous platelet production or function. Unlike red blood cells, which can be efficiently stored at 1-6° C. (mean 4° C.), platelets are irreversibly injured when temperatures repeatedly drop below approximately 20° C. for short periods of time or are kept at less than 20° C. for long periods of time. This injury is termed the “platelet cold storage lesion”. Importantly, this platelet cold storage lesion begins to occur even after brief exposure to temperatures less than 20° C. and is even seen in patients undergoing surgery in which the temperature of the whole body or of parts of the body is decreased to temperatures less than 20° C. and leads to bleeding abnormalities.

FIG. 1 depicts effects on platelets of cooling platelets from 37° C. to 4° C., in accordance with the related art. Exposure of platelets to temperatures less than 20° C. results in structural injury and functional activation of control (normal) platelets. In portion A of FIG. 1, significant morphological changes occur when platelets are cooled from 37° C. to 4° C. as shown by the appearance of filopodia using phase contrast microscopy. In portion B of FIG. 1, temperature dependent activation of platelets is further demonstrated by anti-phosphotyrosine Western Blot analysis of platelets incubated for 30 min at 37° C. (lanes 1, 3) or 4° C. (lanes 2, 4), in the absence (in lanes 1, 2) or presence (in lanes 3, 4) of a membrane-active compound. The blot was stripped and probed for actin as a loading control.

As shown in FIG. 1, key characteristics of this platelet cold storage lesion are: (1) reversible to irreversible morphological change from a discoid cell to spiculated spheres with protruding filopodia, depending on time at temperatures less than 20° C.; (2) irreversible immune-independent microaggregation of platelets (i.e., increased cell:cell interaction); (3) membrane clustering of the glycoprotein GPIb on the surface of platelets resulting in the formation of a neoantigen; and (4) subsequent recognition and phagocytosis by macrophages of the microaggregates and/or neoantigen-expressing platelets upon transfusion into a recipient. In addition, there is a significant reduction in circulation half-life of chilled platelets introduced into a recipient of the chilled platelets. As a consequence of this platelet cold storage lesion, platelets must be stored at 20-24° C. (mean of 22° C.) in order to maintain acceptable function and viability in the transfused patient (see American Association of Blood Banks (AABB) Technical Manual). Unfortunately, maintaining platelets at a mean temperature of 22° C. for prolonged periods of time greatly increases the risk of adverse medical events due to bacterial growth in the platelet product. Current estimates are that 1 in every 3000 platelet units are affected by microbial contamination (see Kleinman S H et al., “Two-year experience with aerobic culturing of apheresis and whole blood-derived platelets”, Transfusion 2006, 46:1787-1794). Risks are associated with transfusion of cellular blood components in Canada (see Transfusion Medicine Reviews, 17:120-163). Because of this microbial risk, platelets can only be stored at 20-24° C. for a maximum of 5 days before they must be destroyed.

Rosiello (International Publication No. WO 2006/044790 A2) discloses a method for the cold storage (−80° C. to 15° C.) of platelets for periods of 3 days to 28 days, by modifying the platelet membrane with a glycan-modifying agent, namely a sugar, a monosaccharide sugar, a nucleotide sugar, sialic acid, sialic acid precursors, CMP-sialic acid, UDP-galactose, and UDP-galactose precursors. Rosiello's method is not practical, however, because it is known that glycosylation (i.e., binding saccharides to proteins and/or lipids) fails to restore the functionality of chilled platelets in vivo.

For example, the inventors of the present invention were present at a seminar at the Center for Blood Research at the University of British Columbia on Apr. 26, 2006 at which Dr. Karin Hoffmeister gave a public presentation entitled “Platelet Glycosylation and the “In and Outs” of Platelet Transfusion” during which Dr. Hoffmeister talked about the problems that had been encountered with glycosylation, said problems including the fact that glycosylation does not protect platelets in chilled platelet concentrates. The results presented at the seminar were also published in a peer-review journal (Blood, 2008, 111: 3249-56).

In addition, Hans Wandall of Zymequest, Inc. gave a public presentation in California at the annual meeting of the California Blood Bank Society on Apr. 28, 2006 in which Hans Wandall substantiated that “glycosylation of platelets does not work, at least after extended storage in the cold and not for larger volumes,” which was confirmed by an attendee of said public presentation by Hans Wandall to an inventor of the present invention via email correspondence on Jun. 22, 2006.

“At a meeting of the American Society of Hematology on Dec. 11, 2006, S. J. Schlichter et al. reported the result of studies relating to galactosylated platelets derived from humans and stored a 4° C. and concluded: “The data show that, following two days of 4° C. storage, the recoveries and survivals of the galactosylated platelets are no different than the non-galactosylated 4° C. stored platelets from the same volunteer. Although the recoveries of the 4° C. stored platelets with and without galactosylation are well-maintained compared to the 22° C. stored platelets, the survivals are markedly reduced as had been previously shown for 4° C. stored platelets (Br J Haematol 1976;34:403).”

Further, very few cryoprotectants are available to store platelets at temperatures below 0° C. Known cryoprotectants are dimethylsulfoxide (DMSO), hydroxyethyl starch (HES), polyethylene glycol (PEG) and glycerol. These cryoprotectants are usually added to plasma containing platelets. Because they either interfere with the blood coagulation mechanism or are toxic, they are usually removed from the platelet suspension before the transfusion (Transfusion Medicine Reviews, 2003, 17: 263-271).

It has been reported that DMSO is the best of these three options as it best preserves platelet morphology and function (Rothwell et al. 2000 Transfusion, 40:988-993). A concentration of 5-6% (w/v) DMSO provides the best results for long-term storage of platelets at −80° C. However, since DMSO is very toxic, removal of the cryoprotective agent present in the thawed cell suspension is a necessary step before transfusion of cryopreserved platelets. This process, currently performed by centrifugation, is labor intensive and negatively affects platelet viability. Indeed, cells washed by centrifugation, which results in a pellet, must be left undisturbed to give the cells time to recover from the washing before resuspension. It has been shown that the release of PF4 and the expression of CD62P are significantly higher with centrifuged platelets, which are signs of platelet activation. In a nutshell, DMSO could be an appropriate cryoprotectant but since it has to be removed from the platelet suspension prior to transfusion, it increases the risk for bacterial contamination and adds a mechanical stress on the platelets.

With respect to the storage and freezing of platelets, it is known in the art that in vitro results are predictive of in vivo functionality (Rothwell et al. 2000 Transfusion 40: 988-993). It is also recognized in the art that it is not necessary for frozen and thawed platelet substitutes to retain 100 percent of in vitro function to have satisfactory results in vivo (Rothwell et al. 2000 Transfusion 40: 988-993).

Thus, there is a need for a method for storing platelets at temperatures below 4° C. such that the stored/thawed platelets have acceptable platelet functionality and viability. There is also a need for a non-toxic cryoprotectant.

SUMMARY OF THE INVENTION

This application relates to method for storing platelets and more specifically to methods for storing platelets at freezing temperatures.

According to a first aspect, the present application provides a method for storing platelets. This method comprises forming at least one modified platelet comprising at least one platelet and at least one polymerated chemical. Each polymerated chemical either comprises (i) a polymer covalently bonded directly to the platelet membrane of the platelet or (ii) a polymer and a linker molecule such that the linker molecule is covalently bonded to the platelet membrane of the platelet and the polymer is covalently attached to the linker molecule. The polymer of each polymerated chemical of each modified platelet may be independently selected from the group consisting of polyethylene glycol (PEG) and a PEG derivative. The method also comprises storing the at least one modified platelet in at temperature of about or below 0° C. or 4° C. for a time period of at least one hour. In an embodiment, the storage temperature is about or below −18° C., about or below −80° C. or about or below −210° C. In another embodiment, the storage time exceeds 12 days. In a further embodiment, the at least one modified platelet in a platelet is also stored in an additive solution. In yet another embodiment, the method further comprises, prior to the formation of the at least one modified platelet, that the at least one platelet is provided from whole blood-derived platelet rich plasma (PRP) platelets, whole blood-derived buffy coat platelets, and/or apheresis platelets. In yet a further embodiment, the polymer of the polymerated chemical consists of PEG and/or a PEG derivative. In yet another embodiment, the at least one modified platelet consists of a plurality of modified platelets, wherein a polymer of a polymerated chemical of a first modified platelet of the plurality of modified platelets consists of a first PEG derivative, and wherein a polymer of a polymerated chemical of either the first modified platelet or a second modified platelet of the plurality of modified platelets consists of a second PEG derivative that differs from the first PEG derivative. In still another embodiment, wherein if after said storing is performed the at least one modified platelet were introduced into a subject in need thereof, then the at least one modified platelet introduced into the subject would restore at least one platelet function in the subject wherein a same number of non-modified platelets introduced into the subject after being stored in the temperature range for the time period would not restore the at least one platelet function. In still a further embodiment, the at least one platelet function is selected from the group consisting of platelet adhesion, platelet activation, platelet aggregation, clot formation, clot retraction, cytokine production and coagulation.

According to another aspect, the present application provides a platelet structure comprising at least one modified platelet at a temperature of about or below 0° C. or 4° C. Each modified platelet comprises a platelet and at least one polymerated chemical. Each polymerated chemical either comprises (i) a polymer covalently bonded directly to the platelet membrane of the platelet or (ii) a polymer and a linker molecule such that the linker molecule is covalently bonded to the platelet membrane of the platelet and the polymer is covalently attached to the linker molecule. The polymer of each polymerated chemical of each modified platelet is independently selected from the group consisting of polyethylene glycol (PEG) and a PEG derivative. In an embodiment, wherein if the at least one modified platelet was subsequently introduced into a subject in need thereof, the at least one modified platelet would restore at least one platelet function in the subject, whereas a same number of non-modified platelets introduced into the subject after being stored in the temperature range for the time period would not restore the at least one platelet function. In a further embodiment, the at least one platelet function is selected from the group consisting of platelet adhesion, platelet activation, platelet aggregation, clot formation, clot retraction, cytokine production and coagulation. In yet another embodiment, the storage temperature of the platelet structure is about or below −18° C., about or below −80° C. and/or about or below −210° C. In still another embodiment, the structure further comprises a platelet additive solution. In yet a further embodiment, the polymer of the polymerated chemical consists of PEG and/or a PEG derivative. In still a further embodiment, the at least one modified platelet consists of a plurality of modified platelets, wherein a polymer of a polymerated chemical of a first modified platelet of the plurality of modified platelets consists of a first PEG derivative, and wherein a polymer of a polymerated chemical of either the first modified platelet or a second modified platelet of the plurality of modified platelets consists of a second PEG derivative that differs from the first PEG derivative.

In yet another aspect, the present application provides a method of treating a condition associated with a reduced platelet function in a subject in need thereof. The method comprising administering at least one modified platelet produced by the method described herein or the platelet structure described herein to the subject, thereby treating the condition in the subject. In an embodiment, the subject is a mammal and, in yet another embodiment, the subject is human or a non-human mammal. The condition can be, but is not limited to, thrombocytopenia, idiopathic thrombocytopenic purpura, thrombotic thrombocytopenic purpura, drug-induced thrombocytopenia, Gaucher's disease, aplastic anemia, alloimmune disorder, fetomaternal alloimmune thrombocytopenia, transfusion reaction, HELLP syndrome, hemolytic-uremic syndrome, chemotherapy-induced thrombocytopenia, dengue and alpha-delta platelet storage pool deficiency. In yet another embodiment, the subject in need thereof has a low platelet count. In still a further embodiment, the method restores normal platelet count in the subject. In yet another embodiment, the at least one modified platelet or the platelet structure has been mixed with a plasma protein prior to its administration to the subject.

In still another aspect, the present application provides use of the modified platelets as described herein for the treatment of a subject in need thereof as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts effects on platelets of cooling platelets from 37° C. to 4° C., in accordance with the related art.

FIG. 2 is a schematic representation of a modified platelet, in accordance with embodiments of the present invention.

FIG. 3 is a flow chart of a method of forming and using modified platelets, in accordance with embodiments of the present invention.

FIG. 4 contrasts mPEG grafted platelets with normal platelets with respect to the respective platelets being chilled, in accordance with embodiments of the present invention.

FIG. 5 depicts modification of platelets with 10 mM BTC-PEG (5000 kDa), in accordance with embodiments of the present invention.

FIG. 6 depicts the effect on morphological changes and microaggregation of cooling and subsequent rewarming of PEG-modified platelets, in accordance with embodiments of the present invention.

FIG. 7 depicts PEGylation of 7 day old platelet concentrates, in accordance with embodiments of the present invention.

FIG. 8 depicts the response of PEGylated platelets to platelet agonists, in accordance with embodiments of the present invention.

FIGS. 9A and 9B depict thromboelastography (TEG) of PEGylated platelets, in accordance with embodiments of the present invention.

FIG. 10 depicts the platelet count (number of cells recovered of PEGylated and untreated platelets) after freezing, storing for up to 12 days at −80° C. and thawing of platelet suspensions.

FIG. 11 depicts the morphology of (A) control fresh platelets, (B) PEGylated fresh platelets and (C) PEGylated frozen/thawed platelets.

FIG. 12 depicts the morphology of (A and D) control untreated platelets, (B and E) PEGylated platelets and (C and F) DMSO-treated platelets. Phase contrast micrographs of (A, B, C) fresh and (D, E, F) frozen/thawed cells are shown.

FIG. 13 depicts flow cytometry results of (A) untreated fresh platelets, (B) fresh PEGylated platelets, (D, F) frozen/thawed PEGylated platelets, (C) fresh DMSO-treated platelets, (E, G) frozen/thawed DMSO-treated platelets. Scatter plots are shown in A, B, C, D and E while CD9 and CD62-P expression are shown in F and G.

FIG. 14 depicts automated optical platelet counts measured with the Advia 120. Optical platelet counts of (A) control fresh untreated platelets, (B) fresh PEGylated platelets and (C) control frozen/thawed untreated platelets and (D) frozen/thawed PEGylated platelets.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method, system, and structure for safely storing modified platelets at temperatures of less than 0° C. subsequent to formation of the modified platelets. The modified platelets are formed by covalent modification of the platelet membrane of the platelets with polyethylene glycol (“PEG”) or derivatives of poly(ethylene glycol) such as methoxypolyethylene glycol (“mPEG”). The covalent modification of the platelets with PEG or a PEG-derivative blocks the adverse effects of the platelet cold storage lesion while maintaining acceptable platelet function and viability (e.g., normal platelet function and viability). As indicated above, PEG has been previously used as an additive in platelet suspensions as a cryoprotectant (Transfusion Medicine Reviews, 2003, 17: 263-271). In the prior art, contrary to what is described herein, PEG has not been covalently linked to the surface of the platelet and interfered with the blood coagulation properties of the thawed platelet suspension. Surprisingly, when PEG or mPEG is covalently attached to the surface of the platelet, as described herein, it does not alter the blood coagulation properties of the modified platelets and act as an excellent cryoprotectant.

As indicate herein, in the art, there are very few techniques for storing platelet solutions at freezing temperatures. As used herein, the term “freezing temperature” refers to a temperature below 4° C. or below 0° C. Cell (e.g. platelet) solutions and suspensions can be stored in freezers having an optimal temperature of −18° C. But generally, in the art, cells (e.g. platelets) suspensions and solutions are stored in industrial freezers having an optimal temperature of −80° C. or in liquid nitrogen having an optimal temperature of −210° C.

Normal in vitro platelet functionality (or platelet function) is defined as full aggregation of platelets in plasma in response to 2 IU/mL thrombin (75-100% increase in light transmission measured by platelet aggregometry test, as illustrated in portion 72 of FIG. 8, described infra) and the potential to recover from mild stress, i.e., recover resting morphology after mild temperature or osmotic stress. Normal in vivo functionality is defined as 67 percent mean post-transfusion recovery (range 50-80%) of stored platelets compared to fresh platelets and 50 percent mean post-transfusion survival (range 30-70%) of stored platelets compared to fresh platelets measured 1 hour or 24 hours after transfusion with both fresh and stored platelets being obtained from the same human being or mammal (Slichter S J et al 2006 “Viability and function of 8-day-stored apheresis platelets”, Transfusion. 46 1763-9; and Murphy S. 2006 “The case for a new approach for documenting platelet viability”, Transfusion. 46 Suppl. 49S-51S).

As it is known in the art, normal platelet function includes various aspects leading to or facilitating clot formation. Normal platelet functions include, but are not limited to, platelet adhesion, platelet activation, platelet aggregation, clot retraction, pro-coagulation and cytokine signaling. Platelets are activated by contacting an activating agent (such as collagen, thrombin, a negatively charged surface, etc). Once activated, platelets release a number of different coagulation factors and platelet activating factors. They also aggregate and adhere to the inner surface of a damaged vessel wall. Because platelets are rapidly deployed to sites of injury or infection, they also modulate the inflammatory process by interacting with leukocytes and by secreting cytokines, chemokines and other inflammatory mediators.

With the present invention, modified platelets can be stored for prolonged periods of time (e.g., more than 12 days) at temperatures less than 0° C. (e.g., −18° C., 80° C. and/or −210° C.). This temperature range significantly inhibits bacterial growth during the cold storage of the platelet suspensions or solutions. This advantage is applicable in the traditional blood banking environment as well as in specific medical interventions involving the transient cooling of the whole or partial body to a temperature of less than 22° C. Thus, the results provided herein satisfy a long-felt, previously unsatisfied need in transfusion medicine for storing platelets under cooling temperature conditions that inhibit microbial growth while maintaining acceptable platelet function and viability.

FIG. 2 is a schematic representation of a modified platelet 60, in accordance with embodiments of the present invention. The modified platelet 60 comprises a platelet 56 and at least one polymerated chemical 59. In one embodiment, the at least one polymerated chemical 59 consists of a plurality of polymerated chemicals 59. The platelet 56 includes a platelet core 47 and a platelet membrane 48 that surrounds the platelet core 47. Each polymerated chemical 59 is covalently bonded to the platelet membrane 48 of the platelet 56. More specifically in one embodiment, each polymerated chemical 59 comprises a linker molecule 61 and a polymer 62, wherein the polymer 62 is covalently attached to the linker molecule 61 and the linker molecule 61 is covalently bonded to the platelet membrane 48 at a bonding site (e.g., at a protein or at a carbohydrate) of the platelet membrane 48. The linker molecule serves to activate the covalent linkage of the polymer 62 to the platelet 56 at the platelet membrane 48.

In an alternative embodiment, a polymerated chemical 89 comprises a polymer 82 covalently bonded directly to the platelet membrane 48 at a bonding site (e.g., at a protein or at a carbohydrate) of the platelet membrane 48. The polymerated chemical 89 is analogous to the polymerated chemical 59, except that the polymerated chemical 89 does not comprise a linker molecule 61, and the polymer 82 is analogous to the polymer 62. Although the discussion infra describes the present invention for the embodiment of the polymerated chemical 59 that comprises the linker molecule 61 and the polymer 62, it should be understood that unless otherwise indicated or otherwise inapplicable, said discussion infra applies likewise to the alternative embodiment of the polymerated chemical 89 that comprises the polymer 82, wherein the polymer 82 is covalently bonded directly to the platelet membrane 48.

The space defined by the at least one polymerated chemical 59 is an envelope 57 that envelopes the platelet 56 due to a “long chain length” of each polymer 62 (i.e., a chain length that has sufficient magnitude to fill the space around itself). The envelope 57 provides a immunocamouflage functionality. A small membrane protein 63 (such as CD9=p24) is covered by the envelope 57 and cannot bind its respective antibody. A large, extended membrane protein 64 (such as CD42b=GPIb) is partially covered by the envelope 57 and reaches through the envelope 57, and can still be recognized and bound by the respective antibody as well as other proteins important for the hemostatic function of platelets. The envelope 57 prevents the formation and/or immunologic recognition of GPIb-clusters and microaggregation.

The polymer 62 in each polymerated chemical 59 is independently selected from the group consisting of polyethylene glycol (PEG) and a PEG derivative. Polyethylene glycol has the formula H(OCH₂CH₂)_(n)OH, wherein n is greater than or equal to 4, with a molecular weight of up to about 20,000 Daltons. Various derivatives of polyethylene glycol may substitute for the H or OH end groups, forming, for example, polyethylene glycol ethers (e.g., PEG-O—R; PEG-O—CH₃; CH₃—PEG-OH); 2,4-dinitrophenyl ethers of PEG), polyethylene glycol esters (e.g., PEG-O₂C(CH₂)₁₄CH₃ ; PEG-O₂CCH₂CH₂CO₂-atropine), polyethylene glycol amides (e.g., PEG-O₂C(CH₂)₇CONHR; mPEG-O₂CCH₂CH₂CONH(CH₃)CHCH₂C₆H₅; PEG-O₂CCH₂CH₂CONHCH₂CH₂—NAD⁺), polyethylene glycol amines (e.g., PEG-NH₂; PEG-NH(CH₂)₆NH₂; PEG-OCH₂CH₂NH₂; mPEG-NH₂), polyethylene glycol acids (e.g., PEG-O₂C(CH₂)₂CO₂H; PEG-O—CH₂CO₂H; PEG-O₂C—(CH₂)₇—CO₂H), polyethylene glycol aldehydes (e.g., PEG-O—CH₂—CHO), and electrophilic derivatives (e.g., PEG-Br; PEG-OSO₂CH₃; PEG-O). Various phenyl moieties can also be substituted for the H or OH of PEG, such as the 2,4-dinitrophenyl ether of PEG mentioned above. The particular polyethylene glycol derivatives listed above are exemplary only, and the invention is not intended to be limited to those particular examples.

The linker molecule 61 may comprise, inter alia, cyanuric chloride, imidazolyl formate, succinimidyl succinate, succinimidyl carbonate, succinimidyl glutarate, N-hydroxysuccinimide, 4-nitrophenol, and 2,4,5-trichlorophenol. The linker molecules listed above are exemplary only, and the invention is not intended to be limited to those particular examples. Any linker molecule capable of covalently attaching to the polymer 62 and mediating the linkage of the polymer to the platelet membrane 48 may be similarly used.

FIG. 3 is a flow chart of a method of forming and using modified platelets, in accordance with embodiments of the present invention. The flow chart of FIG. 3 comprises steps 31-34.

Step 31 prepares at least one platelet (e.g., a plurality of platelets), using any known platelet preparation method such as, inter alia, whole blood-derived platelet rich plasma (PRP) platelets, whole blood-derived buffy coat platelets, or apheresis platelets.

Step 32 forms at least one modified platelet from the at least one platelet prepared in step 31. Each modified platelet conforms to the modified platelet 60 of FIG. 2 and comprises a platelet and at least one polymerated chemical. Each polymerated chemical either comprises a polymer covalently bonded directly to the platelet membrane of the platelet or comprises the polymer and a linker molecule such that the linker molecule is covalently bonded to the platelet membrane of the platelet and the polymer is covalently attached to the linker molecule. The polymer of each polymerated chemical of each modified platelet is independently selected from the group consisting of polyethylene glycol (PEG) and a PEG derivative. Step 32 does not comprise modifying the platelet membrane of the platelets with a glycan-modifying agent, because it is known that glycosylation (i.e., binding saccharides to proteins and/or lipids) fails to preserve the functionality of chilled platelets in vivo as indicated supra. Indeed, it is totally outside of the scope of the present invention to modify the platelet membrane of the platelets with a glycan-modifying agent.

In one embodiment, a polymer of a polymerated chemical of a modified platelet of the at least one modified platelets consists of PEG. For example, the modified platelet 60 of FIG. 2 comprises at least one polymerated chemical, and the polymer of one polymerated chemical of the at least one polymerated chemical may consist of PEG.

In one embodiment, a polymer of a polymerated chemical of a modified platelet of the at least one modified platelet consists of a PEG derivative. For example, the modified platelet 60 of FIG. 2 comprises at least one polymerated chemical, and the polymer of one polymerated chemical of the at least one polymerated chemical may consist of a PEG derivative.

In one embodiment, a polymer of a polymerated chemical of a first modified platelet of the at least one modified platelet consists of a first PEG derivative, and a polymer of a polymerated chemical of either the first modified platelet or a second modified platelet of the at least one modified platelet consists of a second PEG derivative that differs from the first PEG derivative. The preceding embodiment is describing cases in which two different PEG derivatives (e.g., PEG-O—CH₃ and CH₃—PEG-OH) are present in a plurality of modified platelets, wherein the plurality of modified platelets comprise a first modified platelet and a second modified platelet. These two different PEG derivatives are denoted as a first PEG derivative and a second PEG derivative. In one case, both the first PEG derivative and the second PEG derivative are in the first modified platelet. In another case, the first PEG derivatives is in the first modified platelet and the second PEG derivative is in the second modified platelet.

Step 33 stores the modified platelets formed in step 32 in a temperature range below 4° C. or below 0° C. for a time period of at least one hour. In one embodiment, the modified platelets are stored in a platelet additive solution. In one embodiment, the temperature range below 4° C. is a single temperature characterized by an approximately constant value of temperature (e.g., 0° C., 4° C., 10° C., etc.). In one embodiment, the temperature range below 4° C. (or below 0° C.) is, inter alia: from −210° C. to below 4° C., from −80° C. to below 4° C., from −18° C. to below 4° C., etc. The time period of at least one hour may, inter alia: be in a range from 1 day to five days, exceed 5 days, be in a range from more than 5 days to 30 days, be in a range from 30 days to 3 months, exceed 3 months, be in a range from 3 months to 1 year, etc.

The storage of the modified platelets in the temperature range below 4° C. for the time period of at least one hour in step 33 prevents and/or retards microbial growth on the stored platelets during the time period.

In one embodiment, the platelets prepared in step 31 were obtained from a subject such as an animal (i.e., a mammal) and after the storing step 33 has been performed, the modified platelets have a post-transfusion recovery in the animal of 50% to 80%, relative to fresh platelets from the subject, at a post-transfusion time in a range of 1 hour to 24 hours measured from a time of transfusion of the modified platelets and the fresh platelets into the subject. This means that if the post-stored platelets were transfused into the subject, then the percentage of the transfused post-stored platelets that would be recovered in the recipient's circulation is 50% to 80% of the percentage of fresh platelets that would recover in the recipient's circulation at a post-transfusion time in a range of 1 hour to 24 hours measured from a time of the transfusion of the post-stored platelets and the fresh platelets into the subject. In this embodiment, the subject may be the same animal or mammal into which the modified platelets are introduced in step 34 (described infra) or the animal may be another mammal. The modified platelets consist of at least N modified platelets, N being a minimum number of modified platelets necessary for a determination of the post-transfusion recovery to have a statistical error not exceeding a specified threshold percent. The specified threshold percent may be in a range of 1% to 20% or any subset thereof (e.g., 5%, 10%, 5 to 15%, 10% to 20%, 20%, etc.). In this embodiment, the post-transfusion recovery is an acceptable post-transfusion recovery.

In one embodiment, the platelets prepared in step 31 were obtained from a subject such as an an animal (i.e., a mammal) and after the storing step 33 has been performed, the modified platelets have a post-transfusion survival in the animal of 30% to 70%, relative to fresh platelets from the subject, at a post-transfusion time in a range of 1 hour to 24 hours measured from a time of transfusion of the modified platelets and the fresh platelets into the subject. This means that if the post-stored platelets were transfused into the subject, then the percentage of the transfused post-stored platelets that would survive is 30% to 70% of the percentage of fresh platelets that would survive, at a post-transfusion time in a range of 1 hour to 24 hours measured from a time of the transfusion of the post-stored platelets and the fresh platelets into the subject. In this embodiment, the animal may be the same mammal into which the modified platelets are introduced in step 34 (described infra) or the animal may be another mammal. The modified platelets consist of at least N modified platelets, N being a minimum number of modified platelets necessary for a determination of the post-transfusion survival to have a statistical error not exceeding a specified threshold percent. The specified threshold percent may be in a range of 1% to 20% or any subset thereof (e.g., 5%, 10%, 5 to 15%, 10% to 20%, 20%, etc.). In this embodiment, the post-transfusion survival is an acceptable post-transfusion survival.

Step 34 introduces the modified platelets into a mammal after having been stored at a temperature below 4° C. for the time period in step 33. In one embodiment, the subject is a mammal such as a human being. In one embodiment, the subject is a non-human mammal (e.g., dog, cat, horse, rat, etc.).

The modified platelets introduced into the subject in step 34 have a longer circulation half-life in the subject than would a same number of non-modified platelets introduced into the subject after being stored in the temperature range below 4° C. or 0° C. for the time period. The non-modified platelets would be processed in accordance with the flow chart of FIG. 3 except that step 32 is not performed. Thus, the non-modified platelets are prepared as in step 31, stored at temperature below 4° C. or 0° C. for the time period of at least one hour as in step 33, and introduced into the animal as in step 34.

FIG. 4 contrasts mPEG grafted platelets with normal platelets with respect to the respective platelets being cooled, in accordance with embodiments of the present invention.

In the upper portion 5 of FIG. 4, normal platelets 10 comprise glycoprotein (GP) Ib 12 and other membrane proteins 14 inherent to the platelet membrane 16. The normal platelets 10, upon being cooled from 37° C. to 4° C., aggregate with significant shape change wherein the GP Ib 12 form GP Ib clusters 13 at the platelet membrane 16 outer surface in the transformation of the normal platelets 10 to the cooled platelets 20. After introduction of the cooled platelets 20 into a subject, the GP Ib clusters 13 are recognized by CR3 receptors of liver macrophages, which leads to the phagocytosis of the previously cooled platelets 20.

In the lower portion 6 of FIG. 4, the polymerated chemical 59 of FIG. 2 surrounds the platelet 56 to form the modified platelet 60, which is cooled from 37° C. to 4° C., wherein the envelope 57 provides a immunocamouflage functionality that prevents microaggregation of the platelets and reduces platelet shape change upon said cooling. Furthermore, the formation and/or immunologic recognition of GPIb-clusters 13 and other membrane proteins is attenuated due to the envelope 57.

FIG. 5 depicts modification of platelets with 10 mM BTC-PEG (5000 kDa), in accordance with embodiments of the present invention. FIG. 5 comprises normal platelets acting as a control in panels 41-43 and PEG-modified in panels 44-46. Panels 41 and 44 depict the normal and modified platelets, respectively, as fresh platelets or platelets following 24 fours of storage at or above 20° C. Panels 42 and 45 depict the normal and modified platelets, respectively, at 20° C. Panels 43 and 46 depict the normal and modified platelets, respectively, at 4° C.

As seen in panels 41-42 and 44-45, the platelet modification of the modified platelets does not change platelet morphology of fresh platelets or following 24 hours storage at or above 20° C. Furthermore, PEGylation of platelets prevents platelet activation and microaggregation at 4° C., as shown for the modified platelets in panel 46 in comparison with the control platelets in panel 43.

FIG. 6 depicts the effect on morphological changes and microaggregation of cooling and subsequent rewarming of PEG-modified platelets, in accordance with embodiments of the present invention. FIG. 6 comprises panels 67A, 67B, 68, and 69. Panel 67A depicts normal control platelets from platelet concentrates or platelet rich plasma (PRP) fixed at 4° C. Panel 67B depicts microaggregation of normal control platelets from platelet concentrates or platelet rich plasma (PRP) fixed at 4° C. Panel 68 depicts PEGylated platelets in plasma fixed at 4° C. Panel 69 depicts PEGylated platelets rewarmed and fixed at 37° C. after exposure to 4° C. FIG. 6 shows that PEGylation of platelets prevents both significant morphological changes and microaggregation of platelets at or after 30 minutes at 4° C. Furthermore, PEGylated platelets regain normal morphology upon rewarming to 37° C.

As seen in panel 67, the normal control platelets from platelet concentrates or PRP undergo severe morphological changes and form small aggregates when exposed to low temperature (4° C.). Phase contrast microscopy shows long pseudopods and platelet-platelet interactions. As seen in panel 68, PEGylation inhibits severe morphological changes as well as platelet interactions at 4° C. As seen in panel 69, a smooth, resting morphology was restored by incubation at 37° C., which indicates that upon rewarming from 4° C. to 37° C., PEGylated platelets are viable and minor morphological changes caused by chilling are reversible.

FIG. 7 depicts PEGylation of 7 day old platelet concentrates, in accordance with embodiments of the present invention. PEGylation of 7 day old platelet concentrates prevents recognition of platelet surface (e.g., CD9) and activation (e.g., CD62) markers. Shown is anti-CD9 binding to 7 day old platelets (washed before and after reaction with 0 or 10 mM BTC-PEG₅₀₀₀). CD9 antigens were effectively masked on washed PEGylated platelets, which was shown as complete inhibition of FITC-labeled anti-CD9 binding to these platelets. In contrast, control platelets demonstrated ˜ 100% anti-CD9 binding and therefore ˜ 100% of platelets have fluorescently (FITC) labeled antibody bound to them. In FIG. 7 the extent of FITC labeling is shown as % Fluorescence.

FIG. 8 depicts the response of PEGylated platelets to platelet agonists, in accordance with embodiments of the present invention. FIG. 8 shows that PEGylated platelets are fully functional and aggregate in vitro in response to platelet agonists (e.g., thrombin). In portion 71 of FIG. 8, phase contrast microscopy of control platelets in plasma and PEGylated platelets in plasma shows that PEGylated platelets maintain a smooth, resting morphology. In portion 72 of FIG. 8, in response to 2 IU/mL thrombin, control platelets and PEGylated platelets fully aggregate at 37° C. with 1000 rpm stir speed in the aggregometer (ChronoLog). In portion 73 of FIG. 8, control and PEGylated platelets form microscopically very similar thrombin-induced clots demonstrating normal biological function. Aggregates depicted in portion 73 of FIG. 8 came from samples fixed at the end of the experiment shown in portion 72.

FIGS. 9A and 9B depicts thromboelastography (TEG) of PEGylated platelets, in accordance with embodiments of the present invention. The TEG in FIGS. 9A and 9B demonstrates normal platelet function for the PEGylated platelets.

In FIG. 9A, platelet mapping with TEG determines total platelet function. The two symmetric arms show the same results. The parameter definitions are: R: time required for initial fibrin formation); Kc: time to reach a certain level of clot strength (clot kinetics); Angle: speed of fibrin build-up and cross-link (clot strengthening); MA: maximum amplitude: dynamic properties of fibrin and platelet bonding through GPIIb-IIIa.

In FIG. 9B, representative findings obtained with acid citrate dextrose (ACD) anticoagulated control platelets and PEGylated platelets are overlaid on the generic tracing expected for normal whole blood. Both control and PEGylated platelet products fall within the expected ranges; i.e., the speed of fibrin formation and build-up is equivalent and the dynamic properties of fibrin as well as platelet bonding through GPIIb-IIIa/fibrinogen are the same for control and PEGylated platelets.

In another aspect, the present invention provide a method of storing platelet at freezing temperatures wherein the method limits the loss in normal platelet function of the thawed platelet. The platelets are produced by the method described above and illustrated in FIG. 3, except that, in steps 33 and 34, the platelets are stored at a freezing temperature (e.g. less than 0° C., −18° C., −80° C. or −210° C.). Because modified platelets described herein are non-toxic (e.g. do no exhibit cytotoxicity), unlike DMSO-treated platelets, once the modified platelets are thawed, there is no need to wash them prior to their administration to a subject.

In another embodiment, the invention also provides a method of treating a condition associated with a reduced platelet function in a subject in need thereof. The method comprises the step of administering at least one modified platelet produced by the method described herein or the platelet structure described herein. In an embodiment, the modified platelet or platelet structure has been thawed prior to its administration to the subject. In another embodiment, the thawed modified platelet or platelet structure does not need to be washed prior to its administration to the subject.

As used herein, a subject in need thereof is defined as a subject having a low platelet count or dysfunctional platelet activity. Such subjects have a tendency to abnormally bleed. The severity of the dysfunction correlates directly with the severity of the bleeding problem. In the art, it is recognized that normal platelet count in a healthy subject is between 150,000 and 400,000 per mm³ of blood (150−400×10⁹/L). The vast majority of healthy subjects (around 95%) will have platelet counts in this range. As used herein, a healthy subject is either a subject having a normal platelet count or a subject not afflicted by a condition related to decrease platelet function. On the other hand, a subject in need thereof, as used herein, refers either to a subject having a low platelet count (e.g. less than 100×10⁹/L, 80×10⁹/L, 50×10⁹/L or even 5×10⁹/L) or having platelets that do not perform at least one normal platelet function as indicated above. In a further embodiment, the subject in need thereof or the healthy subject is a mammal, a human or a non-human mammal.

The method provided herewith is advantageous for the treatment of subjects afflicted with at least one of thrombocytopenia, idiopathic thrombocytopenic purpura, thrombotic thrombocytopenic purpura, drug-induced thrombocytopenia, Gaucher's disease, aplastic anemia, alloimmune disorder, fetomaternal alloimmune thrombocytopenia, transfusion reaction, HELLP syndrome, hemolytic-uremic syndrome, chemotherapy-induced thrombocytopenia, dengue and/or alpha-delta platelet storage pool deficiency. In some of the conditions listed above, the modified platelets are administered therapeutically to restore a platelet function and ideally, restore all platelet functions. In other conditions (such as chemotherapy-induced thrombocytopenia of subjects suffering from hematologic malignancies), the modified platelets are administered prophylactically because the subjects in need thereof become severely thrombocytopenic but do not (yet) bleed.

The present invention will be more readily understood by referring to the following examples which are given to illustrate the invention rather than to limit its scope.

EXAMPLE 1 Determination of Cold Storage on PEGylated Platelets

Platelet modification with PEG or PEG derivatives is done by mixing a concentration of platelets with chemically activated PEG or PEG derivatives. The concentration of platelets can range from very low counts to very high counts as required by the application; for clinical purposes, a single unit of platelet rich plasma (PRP) should contain at least 5.5×10¹⁰ platelets (see AABB Technical Manual, 12^(th) edition, 1996 American Association of Blood Banks, page 144). Activation of PEG or PEG derivatives is accomplished by chemically modifying one or both terminal reactive groups of PEG or PEG derivatives with a chemical reactive linker group of an associated linker molecule.

Multiple mixing methods can be used to achieve the desired platelet-PEG ratio. In one embodiment, whole blood is collected in ACD (acid citrate dextrose) anticoagulant. Platelet rich plasma (PRP) is prepared from the whole blood by centrifugation (150×g for 12 minutes). Platelet numbers are determined using an automated cell counter. The PRP is mixed with the desired concentration of activated PEG or PEG-derivative using an automated mixing instrument so as to achieve a uniform platelet-PEG ratio. The platelet-PEG mixture is collected and allowed to react for 30 minutes at room temperature. Both the reaction time and temperature can be varied. For example, the reaction time could range from 1 minute to greater than 60 minutes. The reaction time is governed in part by the reactivity of the linker molecule as well as the desired efficiency of the reaction. The temperature should be greater than 20° C. to avoid cold induced injury prior to the protection afforded by the grafted PEG or PEG-derivative.

Following derivatization, the modified platelets can be used as is, or can undergo gentle washing and centrifugation in physiologic solutions (e.g., isotonic saline, ACD, or platelet additive solution that does not contain any plasma proteins). In one embodiment of washing, modified platelets are washed using an excess of a washing buffer consisting of a 1:1 ratio of phosphate buffered saline and ACD at physiologic pH (pH 7-7.8). The platelet-wash solution is mixed gently (e.g., inverting the tube of platelet-wash solution several times) followed by centrifugation at 600 g for 3 minutes. Following washing, the wash supernatant is removed. Platelet counts are determined via automated cell counters and the platelets are resuspended to the desired modified platelet count per unit volume using physiologic solutions (e.g., plasma, saline, platelet additive solutions). At this point, the platelets are suitable for storage at <0° C. and/or experimental or clinical usage. In other embodiments, the washing step is automated using clinical cell washers.

Once the frozen modified platelets are thawed, they can either be used as is, or they can be mixed with plasma (e.g. normal plasma) prior to their transfusion into a subject in need thereof.

In other preparation embodiments, the source of platelets can be whole blood, leukoreduced whole blood, whole blood derived buffy coat platelets or apheresis platelets. Alternatively for non-clinical or veterinary use, a wide range of other platelet preparations (e.g., purified platelets obtained using magnetic bead technology, cell culture and expansion, or via cell sorter technology) can be similarly derivatized. Platelet concentration can also be significantly varied relative to the PEG or PEG-derivative concentration and/or physiologic media.

Depending on the PEG/PEG derivative and the linker group used in the preceding methodology for forming modified platelets, either: (1) the associated linker molecule may remain part of the final structure of the polymerated chemical (as in the polymerated chemical 59 of FIG. 2; e.g., cyanuric chloride activated mPEG); or (2) the linker group may mediate the chemical reaction between PEG/PEG derivative and a protein of the platelet membrane but nonetheless function as a leaving group so that the associated linker molecule is not part of the final structure of the polymerated chemical (as in the polymerated chemical 89 of FIG. 2; e.g., benzotriazole carbonate activated mPEG).

As described supra, the present invention fulfills a long-felt, unsatisfied need in transfusion medicine to store platelets under cooling temperature conditions by inhibiting microbial growth while maintaining acceptable platelet function and viability. The current invention addresses this long-felt, unsatisfied need, by covalently modifying the platelet membrane with PEG or a PEG derivative (FIG. 4). As a consequence of this covalent modification of the platelet membrane, the detrimental effects of cold storage/exposure are inhibited/prevented as evidenced by: maintenance of/return to normal platelet morphology upon transition from 4° to >15° C. (FIGS. 5 and 6); prevention of platelet microaggregation (FIGS. 5 and 6); attenuation of cold storage activation of platelets during storage (FIG. 7); maintenance of normal platelet activation and clot formation upon stimulation (FIGS. 8 and 9); and inhibition and/or attenuation of GPIb clustering and immunologic recognition of platelet surface proteins (FIGS. 4 and 7).

EXAMPLE II Effects of a Freeze/Thaw Cycle on PEGylated Platelets

Platelet Enrichment and Storage

Whole blood was collected in acid citrate dextrose (ACD) and centrifuged at 150×g for 12 min at 26° C. to obtain platelet rich plasma (PRP). Platelet counts were measured with the ADVIA 120 automated hematology analyzer (Beckman Coulter).

The plasma was removed by centrifugation at 500×g for 5 min at 25° C. followed by aspiration of the supernatant and the platelet pellet was resuspended in a platelet storage solution SSP+ (Macopharma, 5 mL total volume). The resultant suspension was split into three aliquots of 1.5 mL each (Control, PEGylated and DMSO-treated samples). An equal volume of SC-mPEG5000 solution in aqueous buffer was mixed with the platelet suspension for a final concentration of 10 mM SC-mPEG5000. For DMSO-treatment the platelet suspension was mixed with 10% DMSO at a 1:1 ratio for a final concentration of 5%. All samples were manually mixed and subsequently kept gently rocked on a shaker for 30 min at room temperature to complete the mPEGylation reaction. Samples were flash frozen in liquid nitrogen and kept at −80° C. for at least 12 hours and up to 6 days before they were thawed. Samples were thawed at room temperature for 30 min with gentle agitation.

Platelet Cell Counts, Morphology, Flow Cytometry and Thrombin Activity

Fresh and thawed samples were analyzed on the ADVIA 120. Aliquots were fixed with paraformaldehyde (2% final concentration) for 1 hour at room temperature for inspection by phase contrast microscopy to determine the platelet morphology. For flow cytometry platelets were washed and resuspended in phosphate buffered saline (PBS). Antibodies to the small surface protein CD9 (anti-CD9-FITC obtained from BD Biosciences), and the activation marker CD62P (anti-CD62P-PE obtained from Immunotech) were added prior to incubation at room temperature for 45 min. The antibody-labeled platelet preparations were then fixed with 0.2% formalsaline.

For light transmission aggregometry, control and PEGylated platelets were centrifuged at 900×g for 10 min at 25° C., and the pellet resuspended in autologous plasma. Platelet suspensions were kept at room temperature and were incubated at 37° C. for 5 min. before the test was started. 450 uL of each sample was stirred at 1000 rpm in the ChronoLog lumi-aggregometer and activated with a final concentration of 2 IU/mL thrombin.

Results

After a freeze/thaw cycle, and as shown on FIG. 10, 90% or more PEGylated platelet cells remained intact and did not lyse, whereas less than 20% of control untreated platelets were recovered. Further, frozen/thawed PEGylated platelets showed a 50% response to 2 IU/mL thrombin compared to fresh control platelets. On the other hand, frozen/thawed untreated control platelets did not show a detectable response to thrombin. During this experiment, all pH values stayed in the physiological range (7.1-7.3).

The PEGylation of platelets also modulated cell morphology of frozen/thawed platelets. As shown on FIG. 11, control fresh platelets (A), PEGylated fresh platelets (B) and frozen/thawed PEGylated platelets (C) display normal platelet morphology whereas as control untreated cells did not withstand the freeze/thaw cycle (data not shown). Fresh PEGylated platelets have normal morphology and do not form micro aggregates.

The protective effects of PEGylation on the morphology of the frozen/thawed cells are similar to those of DMSO. As shown on FIG. 12, untreated control frozen/thawed platelets form microaggregates, lyse and form small cell fragments (FIG. 12D), whereas PEGylated (12E) or DMSO-treated (12F) platelets show typical platelet morphological characteristics.

The high recovery of platelets following freeze/thaw was also measured with flow cytometry (FIG. 13). Before freezing, the platelet populations (count and size) are very comparable as indicated by very similar scatter plots (side scatter SSC vs. forward scatter FSC, FIG. 13A, B and C). After a freeze/thaw cycle, control untreated platelets did not survive. Thawed populations for PEGylated and DMSO-treated platelets are similar but shifted down indicating a reduction in platelet size (FIGS. 13D and E). As shown in FIGS. 13F and G, the number of events in the quadrants marked in grey indicated that platelet activation, measured as CD62-P expression, was lower in PEGylated platelets when compared to DMSO-treated platelets. Further, PEGylation also prevented, before and after the freeze/thaw cycle, CD9 recognition.

The flow cytometry results were confirmed by the optical automated hematology analyzers results generated with the Advia 120 (FIG. 14). The freeze/thaw cycle did not substantially affect the morphology of the PEGylated platelets.

EXAMPLE III Platelet Transfusion in Rabbits Comparing Control, PEGylated and DMSO-Treated Platelets

New Zealand white rabbits first receive gamma irradiation (Cobalt 550cG rads for 10-11 min.). Six days later, rabbits are injected with ethyl palmitate (34% solution, Fluka) and sheep anti-rabbit platelet serum to induce thrombocytopenia (e.g. a platelet count of approximately 20×10⁹ cells/L).

Concentrated platelet suspensions are prepared from platelet concentrates drained into 50 mL conical tubes and centrifuged at 1000×g for 5 min. at 19-20° C. With a syringe, 48 mL of the supernatant is removed and the pellet is resuspended in the remaining supernatant. Eight concentrates are combined to obtain a transfusion dose per rabbit of 20×10¹⁰ platelets in 10 mL. Samples for platelet counts, microscopy and flow cytometry are prepared as described in Example I.

The rabbits are secured and anesthetized. Platelets are transfused through the ear vein. To measure the ear bleeding time, an indicator of platelet function, the other rabbit ear is placed in a 0.9% sodium chloride solution at 37° C. with slow stirring. One hour after platelet transfusion, the ear is cut with a scalpel and the bleeding time is measured until the bleeding stops or to a maximum of 15 min. The bleeding time is determined in duplicates from two different sites. The rabbit is allowed to rest until the next time point for measuring the bleeding time at 3 and 5 hours post transfusion.

While particular embodiments of the present invention have been described herein for purposes of illustration, many modifications and changes will become apparent to those skilled in the art. Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the true spirit and scope of this invention. 

1. A method for storing platelets, comprising: forming at least one modified platelet comprising at least one platelet and at least one polymerated chemical, wherein: each polymerated chemical either comprising a polymer covalently bonded directly to the platelet membrane of the platelet or comprising the polymer and a linker molecule such that the linker molecule is covalently bonded to the platelet membrane of the platelet and the polymer is covalently attached to the linker molecule, and the polymer of each polymerated chemical of each modified platelet being independently selected from the group consisting of polyethylene glycol (PEG) and a PEG derivative, and storing the at least one modified platelet at a temperature of about or below 0° C. for a time period of at least one hour.
 2. The method of claim 1, wherein the temperature is about or below −18° C.
 3. The method of claim 1, wherein the temperature is about or below −80° C.
 4. The method of claim 1, wherein the temperature is about or below −210° C.
 5. The method of claim 1, wherein the time period exceeds 12 days.
 6. The method of claim 1, wherein said storing comprises storing the at least one modified platelet in a platelet additive solution within the temperature range for the time period.
 7. The method of claim 1, further comprising, prior to said forming the at least one modified platelet, providing the at least one platelet from whole blood-derived platelet rich plasma (PRP) platelets, whole blood-derived buffy coat platelets, and/or apheresis platelets.
 8. The method of claim 1, wherein the polymer of the polymerated chemical consists of PEG.
 9. The method of claim 1, wherein the polymer of the polymerated chemical consists of a PEG derivative.
 10. The method of claim 1, wherein the at least one modified platelet consists of a plurality of modified platelets, wherein a polymer of a polymerated chemical of a first modified platelet of the plurality of modified platelets consists of a first PEG derivative, and wherein a polymer of a polymerated chemical of either the first modified platelet or a second modified platelet of the plurality of modified platelets consists of a second PEG derivative that differs from the first PEG derivative.
 11. The method of claim 1, wherein if after said storing is performed the at least one modified platelet were introduced into a subject in need thereof, then the at least one modified platelet introduced into the subject would restore at least one platelet function in the subject wherein a same number of non-modified platelets introduced into the subject after being stored in the temperature range for the time period would not restore the at least one platelet function.
 12. The method of claim 11, wherein the at least one platelet function is selected from the group consisting of platelet adhesion, platelet activation, platelet aggregation, clot formation, clot retraction, cytokine production and coagulation.
 13. A platelet structure comprising at least one modified platelet at a temperature of about or below 0° C., each modified platelet comprising a platelet and at least one polymerated chemical, each polymerated chemical either comprising a polymer covalently bonded directly to the platelet membrane of the platelet or comprising the polymer and a linker molecule such that the linker molecule is covalently bonded to the platelet membrane of the platelet and the polymer is covalently attached to the linker molecule, the polymer of each polymerated chemical of each modified platelet being independently selected from the group consisting of polyethylene glycol (PEG) and a PEG derivative.
 14. The structure of claim 13, wherein if the at least one modified platelet was subsequently introduced into a subject in need thereof, the at least one modified platelet would restore at least one platelet function in the subject, wherein a same number of non-modified platelets introduced into the subject after being stored in the temperature range for the time period would not restore the at least one platelet function.
 15. The structure of claim 14, wherein the at least one platelet function is selected from the group consisting of platelet adhesion, platelet activation, platelet aggregation, clot formation, clot retraction, cytokine production and coagulation.
 16. The structure of claim 13, wherein the temperature is about or below −18° C.
 17. The structure of claim 13, wherein the temperature is about or below −80° C.
 18. The structure of claim 13, wherein the temperature is about or below −210° C.
 19. The structure of claim 13, further comprising a platelet additive solution.
 20. The structure of claim 13, wherein the polymer of the polymerated chemical consists of PEG.
 21. The structure of claim 13, wherein the polymer of the polymerated chemical consists of a PEG derivative.
 22. The structure of claim 13, wherein the at least one modified platelet consists of a plurality of modified platelets, wherein a polymer of a polymerated chemical of a first modified platelet of the plurality of modified platelets consists of a first PEG derivative, and wherein a polymer of a polymerated chemical of either the first modified platelet or a second modified platelet of the plurality of modified platelets consists of a second PEG derivative that differs from the first PEG derivative.
 23. A method of treating a condition associated with a reduced platelet function in a subject in need thereof, said method comprising administering at least one modified platelet produced by the method of claim 1 or the platelet structure of claim 13 to the subject, thereby treating the condition in the subject.
 24. The method of claim 23, wherein the subject is a mammal.
 25. The method of claim 23, wherein the subject is human.
 26. The method of claim 24, wherein the mammal is a non-human mammal.
 27. The method of claim 23, wherein the condition is selected from the group consisting of thrombocytopenia, idiopathic thrombocytopenic purpura, thrombotic thrombocytopenic purpura, drug-induced thrombocytopenia. Gaucher's disease, aplastic anemia, alloimmune disorder, fetomaternal alloimmune thrombocytopenia, transfusion reaction, HELLP syndrome, hemolytic-uremic syndrome, chemotherapy-induced thrombocytopenia, dengue and alpha-delta platelet storage pool deficiency.
 28. The method of claim 23, wherein the subject in need thereof has a low platelet count.
 29. The method of claim 28, wherein the method restores normal platelet count in the subject.
 30. The method of claim 23, wherein the at least one modified platelet or the platelet structure has been mixed with a plasma protein prior to its administration to the subject. 